1. Field of the Invention
The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device which emits a laser beam of a variable frequency depending upon a current applied to the device.
2. Related Background Art
Demands for a semiconductor laser device in a light communication field and a optical information processing field have been rapidly increasing including more versatile functionality. A semiconductor laser device having a variable wavelength is one of the functions. For example, when information is to be recorded and reproduced by irradiating a laser beam to a medium such as an optical card or an optical disk, writing by a reproduced light is prevented by setting an output of the reproduced light to be lower than a recording light. By setting the wavelength of the reproduced light to a low sensitivity area of the medium, the writing can be prevented without significantly reducing the output of the reproduced light and the information is reproduced with a high S/N ratio.
For the above requirement, a variable wavelength semiconductor laser device which uses a high order quantum level of a multi-quantum well (MQW) has been proposed. FIG. 1 shows an energy band in a vicinity of a light emission region in the prior art semiconductor laser device. A light emission laser 23 has a MQW structure having well layers 22 and barrier layers 21 alternately laminated. Clad layers 20 are provided on the opposite sides of the light emission region 23 and the barrier layers 19 to form an optical wave guide structure 24. When a current is injected to the semiconductor laser element, electrons 25 are stored at an energy level E.sub.0 and recombine with holes 26 so that a light between quantum levels of n=0 (n: principal quantum number) is generated (wavelength .lambda..sub.1). As the injection current increases, a carrier density of an energy level E.sub.1 increases so that a light between quantum levels of n=1 is generated (wavelength .lambda..sub.2) by recombination. In this manner, lights of different wavelengths are generated from one device.
However, the prior art variable wavelength semiconductor laser element has the following problems.
(I) In order to generate lights of different wavelengths, it is necessary that absorption loss and mirror loss are significantly larger than normal, and hence the device efficiency is low.
(II) Since different quantum levels are merely used, the difference between the generated wavelengths is several tens nm at most.
(III) Since it is necessary to form a quantum well having two or more levels, one-level quantum well cannot be used even if such a device improves a characteristic.
IV It is difficult to switch the wavelength. (Namely, once a light of a wavelength .lambda..sub.2 is generated, it is difficult to stop the generation of a light of a wavelength .lambda..sub.1.)